1. Field of the Invention
The invention is directed to novel methods and compositions useful for improving the performance of electrophoretic displays.
2. Description of Related Art
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. It was first proposed in 1969. The display usually comprises two plates with electrodes placed opposing each other, separated by spacers. One of the electrodes is usually transparent. An electrophoretic fluid composed of a colored solvent with charged pigment particles dispersed therein is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side or the other causing either the color of the pigment particles or the color of the solvent being seen from the viewing side.
There are several different types of EPDs. In the partition type EPD (see M. A. Hopper and V. Novotny, IEEE Trans. Electr. Dev., 26(8):1148-1152 (1979)), there are partitions between the two electrodes for dividing the space into smaller cells in order to prevent undesired movement of particles, such as sedimentation. The microcapsule type EPD (as described in U.S. Pat. Nos. 5,961,804 and 5,930,026) has a substantially two dimensional arrangement of microcapsules each having therein an electrophoretic composition of a dielectric fluid and a suspension of charged pigment particles that visually contrast with the dielectric solvent. Another type of EPD (see U.S. Pat. No. 3,612,758) has electrophoretic cells that are formed from parallel line reservoirs. The channel-like electrophoretic cells are covered with, and in electrical contact with, transparent conductors. A layer of transparent glass from which side the panel is viewed overlies the transparent conductors.
An improved EPD technology was disclosed in co-pending applications, U.S. Ser. No. 09/518,488, filed on Mar. 3, 2000 (corresponding to WO 01/67170), U.S. Ser. No. 09/606,654, filed on Jun. 28, 2000 (corresponding to WO 02/01281) and U.S. Ser. No. 09/784,972, filed on Feb. 15, 2001 (corresponding to WO02/65215), all of which are incorporated herein by reference. The improved EPD cells are prepared by microembossing a layer of thermoplastic or thermoset resin composition coated on a first substrate layer to form the microcups of well-defined shape, size and aspect ratio. The microcups are then filled with an electrophoretic fluid and sealed with a sealing layer. A second substrate layer is laminated over the filled and sealed microcups, preferably with an adhesive layer.
To reduce irreversible electrodeposition of dispersion particles or other charged species onto the electrodes (such as ITO), a thin protection or release layer may be coated on the electrodes. The protective layer improves the performance of the display, including an increase in display image uniformity and longevity. In addition, a faster electro-optical response has been observed in displays with a protective layer.
However, the thin protective layer method also has disadvantages. For example, the use of a protection or release layer on electrodes tends to result in deterioration in contrast ratio and bi-stability of the EPD. A higher Dmin (or a lower degree of whiteness or % reflectance) in the background particularly at low driving voltages is also typically observed in EPDs with coated electrodes.
Accordingly, there is a need for more effective methods to improve the response rate and image uniformity and also to reduce irreversible electrodeposition of dispersion particles or other charged species onto the electrodes.